There are many applications which could benefit from a reduced size actuator, but which can induce a significant stroke and apply a significant force.
Motors are most commonly used as actuators for generating large strokes and forces. For many miniaturized applications, even small motors are too large, heavy, noisy and expensive. Moreover, miniature motors suffer from lower efficiency.
Shaped memory materials (SMMs), especially shape memory alloys (SMAs), are able to provide significant force and stroke when heated beyond their specific phase change temperature. Even if the dimensions of the material are small, the force and stroke delivered are, relative to these dimensions, very high and accurate, over a very long period of time and after many switching operations.
The use of shape memory materials to provide an actuation signal, dependent on temperature, has thus been investigated. For example, US2014/0007572 discloses the use of a shape memory alloy wire which contracts upon an increase in temperature above the phase-change temperature of the material, thereby engaging the actuator. Upon temperature decrease and transition of the wire back to the low temperature phase, restoration of the original length of the wire, and hence resetting of the actuator, is achieved through the use of additional bias springs which stretch the wire back to its low temperature length.
The requirement for external mechanical biasing to reset the shape of the actuator is a disadvantage which follows from the fact that when there is a temperature decrease, the phase changes back to the original phase, but the shape does not. Thus, before the actuator can be used again, after a temperature decrease, an external actuation must be initiated to reverse the shape change of the SMM.
External mechanical biasing introduces significant bulk, cost and manufacturing complexity to an actuator device, often undermining many of the key advantages which an application of SMAs can deliver. Moreover, the use of permanent mechanical bias springs reduces the effective stroke of the actuator.
Desired therefore is a miniaturized actuator device utilizing SMA materials in order to provide significant actuation force, having bi-directional capability, but without the requirement for bulky or complex reverse biasing mechanisms in order to reset the actuator in between strokes.